msbuild warnings - MSB3246 & NETSDK1049

I'm working on migrating from packages.config to PackageReference format for multiple csproj files. Restore and build succeeds but with hundreds of warnings. These warnings were not showing before my PackageReference changes to csproj files. Please let me know if you could guide on resolving these.

warning NETSDK1049:

Resolved file has a bad image, no metadata, or is otherwise inaccessible. An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B) C:\Users\User_Name\.nuget\packages\Package_Name\Package_Version\lib\net45\AssemblyName.dll

warning MSB3246:

Resolved file has a bad image, no metadata, or is otherwise inaccessible. Could not load file or assembly 'AssemblyName.dll' or one of its dependencies. The module was expected to contain an assembly manifest.